This invention relates to metal composite friction materials and more particularly metal composites comprising a metallic continuous phase or matrix having dispersed therein discrete non-metallic particulate components, said composites being suitable for use as friction materials, and to a method for the preparation thereof. The composites may be die-cast to form friction elements such as brake pads and the like, or cast onto the driven member of a clutch mechanism to provide the friction element therein.
Asbestos has long been the principal component of friction elements used in the clutch and brake assemblies where severe operating temperatures and pressures occur. However, due to the accumulating evidence that asbestos may be carcinogenic to man, major efforts have been made to develop alternative friction materials which do not depend upon asbestos for their frictional properties and resistance to extreme temperatures. The more recent friction materials developed as replacements for the asbestos based compositions have employed a variety of glass fibers and infusible organic fibers embedded in a heat-curable organic binder. To provide increased mechanical strength and resistance to disintegration in response to centrifugal forces (burst strength), these materials have included therein continuous glass and/or infusible organic fiber. Fabrication of these materials, particularly when used in the manufacture of clutch facings, has been accomplished by first forming a continuous tape or strand, winding the tape to form a disc preform, then molding and curing the preform under heat and pressure to provide the clutch facing, as is shown for example in U.S. Pat. No. 4,244,994. Although suitable asbestos-free friction elements are provided by such processes, the complexity and cost factors are somewhat limiting. Further, the presence of organic binders precludes the use of these materials under extremely severe conditions wherein very high temperatures are encountered. The thermal deterioration of the binders in severe environments and/or under abuse results in inferior frictional characteristics and often results in increased wear.
Various sintered metal and ceramic compositions have been developed as frictional materials for use under severe conditions. In general, these materials comprise sintered lead bronzes or iron powders with friction reinforcers and dry lubricants. Among the additives commonly employed with the powdered metals are graphite, quartz, corundum, aluminum oxide, silica, mullite and the like. The sintered compositions or composites are generally formed by blending the powdered metals with the powdered or particulate non-metallic component, then compressing the powder in a mold at pressures of from 10,000 to 100,000 psi to provide a pressed wafer. The wafer is then sintered under pressure at elevated temperatures for extended periods to fuse the metallic component and entrap the non-metallic ingredients. The compressed wafer prior to being sintered is fragile and easily broken if mishandled during manufacture. Further, the sintered composite friction element is generally porous and has rather low mechanical strength. It has therefore been necessary to provide additional strengthening means in order to overcome these inherent deficiencies. For example, the friction element may be attached to a steel backing, normally by brazing, prior to the sintering operation to overcome the fragility of the compressed wafer and to add strength to the sintered friction element. Various organic binders such as coal tar pitch may be added to provide green strength to the wafer and thus avoid breakage of the fragile wafer during the sintering operation. Alternatively, monolithic friction elements having a layered structure comprising a first friction layer with appropriate friction and wear properties and second layer united thereto as a backing of adequate mechanical strength may be made by forming the element simultaneously from two powdered layers.
The sintering process thus lends itself to the production of a wide variety of metallic composites for use in most frictional materials applications. However, the weight of sintered metallic friction materials as compared with those formed from glass fiber and resin binders, together with the high cost of manufacturing the sintered composites have restricted their use and have mitigated against the wide spread adoption for use in large volume applications such as in passenger vehicles.
A simplified, low cost method for the manufacture of lighter weight metallic composite friction materials would thus advance the development of asbestos-free friction elements.